EXPERIMENT 4 THE N BOTTLE PROBLEM INTRODUCTION The purpose of this experiment is to use your knowledge about acid base and precipitation reactions that occur in aqueous solutions to identify the ionic substances present in a set of solutions These solutions will be labeled A through G and you will be given the list of cations and anions that are present All the solutions that you will be given are clear hence you can already conclude that only cation anion combinations that do not react with each other have been selected Recall that when a cation and anion are mixed a reaction occurs only if a precipitate is observed or a covalent compound such as water or a gas is formed As a simple example suppose that you are given three clear solutions X Y and Z and are told that the solutions may contain the cations Na H3O and Ag each ion used once and the anions CO3 and Br Since the solutions are clear you may conclude that one solution does not contain AgBr since 2 NO3 AgBr is insoluble in water and would form a precipitate Likewise one solution does not contain Ag2CO3 since this compound is also insoluble Thus one solution must be AgNO3 In a similar fashion you know that none of the unknowns can contain both H3O and CO3 2 because hydronium ion and carbonate ion react to form carbon dioxide gas and water according to the following equation CO aq 2 H3O aq H2CO3 aq H2O l CO2 g 3 H2O l 4 1 Thus the others must be H3O Br and NaNO3 You could identify which is which by systematically mixing 0 5 mL pairs of solutions and observing what happens Suppose you mix X with Y and observe the formation of a pale yellow precipitate Since Na and H3O compounds are soluble either X or Y must contain AgNO3 and the reaction which occurs must be one of the following 2 Ag aq CO3 2 aq Ag aq Br aq Ag2CO3 s AgBr s 4 2 4 3 Suppose you now mix X with Z and also observe a yellow precipitate Then one of the above two reactions must have occurred this identifies X as AgNO3 aq The identity of Y and Z is still unclear If you mix Y and Z you will observe bubbles due the reaction described in EQUATION 4 1 We have no evidence at this point about Na and H3O A flame test could be used to differentiate between these ions Na gives a bright orange yellow flame test while H3O gives no discernable color in a flame In addition testing the unknown solutions with litmus paper can aid in our identification of the unknowns as the presence of H3O will cause blue litmus to turn red Among the pieces of information you will need for this experiment is a set of solubility rules Several statements of the rules are possible However the set of rules given below is particularly useful because the rules are listed in hierarchical order that is Rule 1 takes precedence over Rule 2 and so on Experiment 4 4 1 23 1 All Li Na K and NH4 salts are soluble 2 NO3 C2H3O2 and ClO4 salts are soluble 3 Pb2 and Hg2 2 salts are insoluble 4 SO4 2 salts are soluble except for those of Ca2 Sr2 and Ba2 5 Ag salts are insoluble 6 Cl Br and I salts are soluble 7 CO3 2 S2 O2 PO4 3 and OH salts are insoluble Thus for example using these rules we can predict that NaOH is soluble Rule 1 takes precedence over Rule 7 that Pb C2H3O2 2 is soluble Rule 2 takes precedence over Rule 3 and that AgBr is insoluble Rule 5 takes precedence over Rule 6 TECHNIQUE For mixing solutions you can use small micro test tubes and plastic disposable pipets with graduated volume markings on the barrel The precise amounts of solutions you mix are not crucial a good idea would be to use about 0 5 mL of each You can hold the tube and flick it with your finger to achieve mixing It is never a good idea to put your finger over the mouth of the test tube and shake it After mixing solutions and observing any reaction you may discard the contents of the test tube into the designated waste container You may work in pairs and discuss the experiments and interpretations with your partner However each person must submit an independent lab report Alternatively you may work alone if you so choose EQUIPMENT NEEDED Alcohol burner red and blue litmus paper small test tubes CHEMICALS NEEDED nichrome wire medium test tubes glass stirring rod unknown bottles labeled A G These unknowns contain the following cations and anions Ag Ba2 Co2 H3O Li Na Sr2 and Cl CO3 2 Each solution contains a different 2 OH NO3 SO4 cation but more than one solution can contain the same anion Recall that a salt solution must contain both a cation and an anion PROCEDURE In order to help you interpret the results you should prepare a chart detailing what you predict will happen when the various unknown solutions are mixed This can be accomplished using the solubility rules given in the Introduction For example to determine whether Na will form a precipitate with CO3 2 look for the first mention of either ion in the solubility rules Since Rule 1 states that all Na salts are soluble Na does not form a precipitate with CO3 2 and you should write NR no reaction on your chart Experiment 4 4 2 Litmus Tests Flame Tests solution Why If a reaction does occur for the mixing of another pair of ions write the formula of the reaction product A template for this chart can be found after the report sheet in this handout Turn in the copy of this chart to your instructor who will grade it as a portion of your lab score for this experiment and return it to you before the end of the lab period Keep in mind that the mixing of some ions will result in the formation of a product that is a liquid or a gas such as in EQUATION 4 1 Also recall the important reaction that occurs between H3O and OH Obtain about 10 mL of each solution A G about of a test tube in separate clean medium test tubes Label the test tubes carefully Test the acidity or basicity of each solution by placing a drop of the solution onto strips of red and blue litmus paper An acidic solution H3O present will turn blue litmus red and a basic solution OH or CO3 2 present will turn red litmus blue Obtain a piece of nichrome wire attached to a glass stirring rod The wire should have a small loop on the end if it does not wrap the end around a pen or pencil You will want to be sure that the wire itself is free of any material that could color the flame Thorough rinsing of the wire with tap should …